Maag has developed a complete portfolio of external gear pumps that have been designed to offer the versatility, strength, reliability, safety and flexibility needed for use in chemical-handling applications
By Claudio Bonafede
Introduction
Industrial manufacturing processes that involve the use or creation of chemicals are some of the most critical in the world. They are also some of the most intricate and complex. They typically consist of a series of unit operations that must perform precisely, harmoniously, reliably and safely throughout the entire manufacturing process. Within these unit operations, consistently achieving the safe and efficient handling and transfer of raw materials or finished chemicals along the production line is a daily front-of-mind concern for plant operators in a wide range of industries. To complete these crucial liquid-handling tasks, plant operators rely on industrial pumps. The pumps they use must be able to meet the specific handling characteristics of a dizzying array of fluids. Among these varying characteristics are viscosities from water to molasses-like, wide temperature and pressure ranges, and differing chemical makeups, which can include corrosive, abrasive and potentially hazardous compounds. While certain types of pumps have gained acceptance as the best technology in chemical-handling and manufacturing applications – centrifugal designs being the most prominent example – there are other options to consider. This article will illustrate how one type of pump technology – positive displacement (PD) external gear pumps – can offer the consistency, reliability, efficiency and liquid-handling safety to become a first choice for plant operators where chemicals are either made or used in the creation of other end-products.
The Challenges For Gear Pumps
While gear pumps have been proven in the field to excel in many unique applications, before they can be selected for use, the operator must be fully aware of the characteristics of the production process, as well as the makeup of the raw materials or final chemicals that will be used to complete the process. At their most basic, gear pumps perform well in chemical-based applications because their mode of operation is simple. The liquid enters through the suction side of the pump and into the openings created by the un-meshing of the gear teeth. The liquid is then conveyed through the cavities around the outside of the gears to the pump’s discharge side. The re-meshing of the gear teeth drives the liquid out of the cavities and into the discharge port. Since gear pumps are PD pumps, the flow rate is theoretically independent of the pressure variations that can occur upstream and downstream of the pump. However, some volumetric inefficiencies, or product “slip,” are common in all types of PD pumps. That means that if the gear pump is to achieve high differential pressures at the required flow rate, it must overcome the inherent internal slip. There are four main areas of slip: 1) between the gear shaft and bearings; 2) between the gear tips and the pump housing; 3) between the gear faces and the bearing (or bushing) faces; and 4) between the meshing gears. To eliminate these slip areas, optimizing the clearances between all of the aforementioned parts is of paramount importance with lower clearances resulting in reduced slip. The clearances are usually defined by several factors, including temperature, viscosity and the pump’s materials of construction. However, some slip is actually required to ensure proper operation of the pump. This designed-in slippage allows the creation of the amount of lubrication film and hydraulic lift that is required to support the gear shafts in the bearings. The proper selection of the materials that make up the pump is essential. In order to meet a wide range of requirements, several different options of wetted materials that are compatible with a wide variety of corrosive, abrasive and sensitive fluids are readily available. In high-temperature applications, the selection and the design of the pump becomes even more important as different materials have different rates of thermal expansion, which means that the clearances are constantly changing. As previously mentioned, the clearances need to be as small as possible in order to handle the pressure and deliver the required flow rate. In high-temperature applications, different thermal-expansion rates can lead to pump seizure when brought up to process temperature. For this reason, the pumps may not always perform efficiently at temperatures they were not designed for. Liquid viscosity is another factor that must be carefully considered. If the clearances are too small, the internal flow of the pumped fluid will be limited and can lead to bearing overheating due to lack of lubrication. If the clearances are too large, the lubrication-fluid film can collapse, leading to insufficient support and lubrication of the shafts. In those applications where high-viscosity products are involved, the first concern is ensuring that the fluid is flowing into the pump. Special inlet designs can help facilitate liquid flow. Additionally, in high-viscosity applications, the pump must be turning slow enough to effectively allow the fluid to fill the opening cavities of the un-meshing gear teeth. The hydrodynamic performance of the bearings is also directly linked to the internal clearances and an incorrect selection could lead to premature wear of the bearings and failure of the pump. Two final factors that need to be considered in order to optimize gear-pump performance is the design of the gear teeth and, especially with highly viscous products, the sizing of the gear shaft. The design of the gear teeth is essential because the torque applied on the gear shaft and the shearing forces on the gear teeth increase with increasing viscosity and differential pressure. The gear shafts also need to be strong enough to absorb and transmit the torque that is created by the drive. Gear teeth and shaft design become even more important when these conditions are combined with high temperatures since the components will become more susceptible to failure because of a decreasing modulus of resistance.
Meeting The Need
For more than 90 years, Maag, Oberglatt, Switzerland, a Dover company, Downers Grove, IL, USA, has been considering the challenges inherent in the manufacture and handling of chemical compounds and finished products, and has developed a complete family of external gear pumps that have been designed to overcome those challenges.
Conclusion
The list of demands related to chemical processing is long and critical. These demands will only be satisfied through the selection and use of the proper pump technology – one that is able to meet the needs of varying pressure, temperature, viscosity and compatibility concerns. Maag, with its long history of gear-pump design and manufacture, as well as its extensive installed base, has worked for many decades to identify and develop the best solutions when external gear pumps are the best choice in crucial chemical-handling and manufacturing processes.
About the Author:
Claudio Bonafede is Managing Director of Maag Italy and is based out of Maag’s facility in Rozzano, Italy. He can be reached by claudio.bonafede@maag.com.
For more than 90 years, Maag, a Dover company, has developed gear-pump technologies that are designed for use in the handling of polymer melts, chemicals and lubricants with varying viscosities and at differing temperatures and inlet/discharge pressures.
For more information on Maag Pump Systems or Dover, please go to maag.com or dovercorporation.com.
